Neurological monitoring device test circuitry

ABSTRACT

Circuitry for use with a brain wave monitoring device is provided for selectively monitoring the impedance of the current paths in the patient&#39;s scalp concurrently with brain wave monitoring, and circuitry for self-testing the system for proper functioning.

DESCRIPTION OF THE INVENTION

The present invention is directed to circuitry for testing the operation of a neurological monitoring device. The present invention is related to the subject matter of co-pending U.S. patent application Ser. No. 258,588, by Callahan, Rhodes and Johnson, for "Neurological Monitoring Device", filed Apr. 29, 1982. Specifically, the test circuitry of the present invention is adapted to be used with the neurological monitoring device disclosed in the co-pending application. The monitoring device is thus set forth in the description of the present invention for the sake of completeness.

Electroencephalographs (EEG's) display and record voltage potentials produced by the brain. These voltages are developed by applying electrodes to the scalp of a patient, and by applying the outputs of the electrodes to an amplifying and recording apparatus, a record of the brain waves may be produced. The brain wave voltages are very small, on the order of 50 microvolts, and the degree of amplification required to produce a discernable record is therefore high.

The brain wave voltage potentials vary in amplitude and frequency. For example, "alpha" waves have a voltage of approximately 50 microvolts and have a frequency which varies around 9-10 cycles per second. The alpha waves are typically found in the occipital area of the scalp. "Beta" waves on the other hand, occur mainly over the frontal and temporal lobes and are of even lower voltage and higher frequency than the alpha waves. In addition, certain other waves may be found in normal subjects.

The usefulness of electroencephalograph monitoring during operations is well known. By monitoring the patient's brain waves during an operation, the patient's physical state may be determined by proper interpretation of the EEG. EEF monitoring is presently recommended for cerebrovascular surgery, cardiopulmonary by-pass and deliberate hypotension, and as a measure of anasthetic depth.

In order to insure proper readings from the monitoring device, it is important that the electrodes attached to the patient's scalp have proper impedance properties. In the event that one or more of the electrodes is not properly attached, the impedance between the electrode and the patient's scalp will be high and the readings taken therefrom will be distorted and unreliable.

Attempts have been made at monitoring the impedance of electrodes attached to patient's scalp. For example, Maynard in U.S. Pat. No. 3,699,947, employs an RC circuit at the input of the monitoring device. A relatively high frequency AC source is applied in common to the electrode and the input of the monitoring device. Depending upon the impedance of the current path through the patient's scalp, the RC network will act either basically resistive or capacitive. If capacitive, the signals applied to the input of the device will be shifted 90 relative to the source AC signals. The signals injected into the device are thus collected through a frequency selective amplifier and logically compared to the AC signal source in an AND gate, to thus provide a signal indicative of the impedance of the current path.

In another system, Pisarski et al., U.S. Pat. No. 3,910,258, detects a loose electrode by assuming that the same will act as an antenna. A circuit is provided for detecting 60 cycle hum with a filter to indicate the displaced electrode.

It is an object of the present invention to provide an improved technique for checking the impedance of the current paths through the patient having electrodes placed on the scalp during brain wave monitoring.

It is a further object of the present invention to provide a system for automatically self-testing a brain wave monitoring device to insure that the monitoring device is operating properly.

Generally, circuitry is provided for use with a device for monitoring brain wave signals produced by electrodes placed on a patient's head, for detecting the impedance properties of at least one current path on the patient's head. The circuitry includes a device for providing a time varying signal having an amplitude and frequency substantially greater than the amplitude and frequency of brain wave signals to at least one of the electrodes, circuitry for measuring the level of the signal injected into the brain wave monitoring device, and comparator circuitry for comparing the output of the measuring device to a predetermined level to provide a signal when the level of one of the injected signal and the predetermined level is higher than the other, and a display device for indicating the existence of the signal from the comparator circuitry to thereby enable the impedance of the current path to be monitored concurrently with the use of the brain wave monitoring device.

Circuitry is also provided for use with a device for monitoring brain wave signals produced on the output of electrodes placed on a patient's head, for testing the operation of the device, the circuitry including a device for applying a time varying signal having a frequency within the approximate range of brain wave frequencies and an amplitude substantially above the range of brain wave signal amplitudes. Circuitry for connecting the output of the electrodes and the time varying signal in common to the device is provided for providing a common input signal having an amplitude substantially determined by the amplitude of the time varying signal. Finally, circuitry is provided for reducing the amplitude of the common input signal to substantially that of the known range of brain wave signal amplitudes.

These and other objects of the invention will be described with reference to the following drawing figures of which:

FIG. 1 is a front view of the brain wave monitoring device in accordance with the invention of the co-pending application;

FIG. 2 is a block diagram illustrating the overall system organization in the brain wave monitoring device in accordance with the invention of the co-pending application;

FIG. 3 is a block diagram illustrating the contents of a single Power Spectrum Analyzer used in accordance with the brain wave monitoring device of FIG. 2;

FIG. 4 is a schematic diagram illustrating the circuitry contained in the Power Spectrum Analyzer employed in the brain wave monitoring device in accordance with the invention of the co-pending application;

FIG. 5 is a schematic diagram illustrating the circuitry in accordance with the persent invention which tests the impedance of the current paths through the patient's head during the monitoring of the brain waves, and associated circuitry for providing a self-test of the brain wave monitoring device in order to determine whether the device is functioning properly; and

FIG. 6 is a schematic diagram illustrating the circuit arrangement for driving the LED displays employed in the brain wave monitoring device.

FIG. 1 is an illustration of the front panel of the brain wave monitoring device. ON/OFF switch 2 and potentiometer control 4 are disposed in the lower left hand corner of the panel. In the upper left hand corner is a diagram 6 of the patient's head illustrating the placement of the bipolar electrodes. Associated with diagram 6 are four LED indicators 8a-8d, which are used to determine the impedance properties of the current paths in the patient. To the right of the diagram 6 is the brain wave power indicator comprising a pair of vertically disposed linear LED arrays 10a and 10b arranged side-by-side. To the right of the power indicators are frequency indicators comprising a similer pair of vertically disposed linear LED arrays 12a and 12b also arranged side-by-side. A push button test switch 14, earphone output jack 16 and associated toggle switch 18, and recorded output jack 20 are provided on the right hand portion of the front panel.

FIG. 2 is a block diagram of the system organization of the brainwave monitoring device. Bipolar inputs 22 and 24 from the electrodes placed on the left and right portions of the patient's head are applied to left and right Power Spectrum Analyzers 26 and 28, respectively. The outputs from Power Spectrum Analyzer 26 are applied to power LED display 10a and frequency LED display 12a, while the outputs from Power Spectrum Analyzer 28 are applied to power LED display 10b and frequency LED display 12b. Headphone circuitry 30 receives a signal from one of Power Spectrum Analyzers 26 and 28 via toggle switch 18. System test circuit 36, having test button 14, is connected to the bipolar inputs 22 and 24 of analyzers 26 and 28, respectively. Finally, Power Spectrum Analyzers 26 and 28 provide signals to impedance test circuits 32 and 34, respectively. Impedance test circuit 32 provides an output to LED indicators 8a and 8b, while impedance test circuit 34 provides signals to LED indicators 8c and 8d.

The contents of Power Spectrum Analyzer 28 will be discussed in more detail with reference to FIG. 3. The details of Analyzer 26 are identical to that of Analyzer 28 and have therefore been omitted. The bipolar inputs 24 are applied to input stage 35 which functions to filter the signals and provide a high input impedance path therefor. The bipolar outputs of stage 35 are applied to differential amplifier 38 which also functions to filter the signals input thereto and provides a differential amplification of the bipolar signals to produce a signal output signal. This output signal is applied to Butterworth filter 40, the output of which is applied to a second Butterworth filter 44 via amplifier 42. Butterworth filters 40 and 44 function to provide the appropriate pass band for the system. The output of filter 44 is applied on the one hand to detector 46 and on the other hand to a frequency-to-voltage converter 48, the output of the latter being applied to frequency display 12b, FIG. 2. The output of detector 46 is applied on the one hand to earphone circuitry 30 and on the other hand to amplifier 50, the output of amplifier 50 being applied to the power display 10b, FIG. 2.

The contents of each of the blocks illustrated in FIG. 3 will be discussed with reference to FIG. 4. Again, the details of Analyzer 26 are identical to those of Analyzer 28 and have therefore been omitted. The bipolar input from the electrode is applied to input stage 35 comprising a pair of high pass filters C1/R1 and C2/R2 and operational amplifiers A1 and A2. Specifically, the positive bipolar terminal is applied to the noninverting input of amplifier A1 via capacitor C1, the output of amplifier A1 being fed back to the inverting input thereof. Similarly, the negative terminal of the electrode is applied to the noninverting input of amplifier A2 via capacitor C2, the output thereof being fed back to the inverting input. The noninverting inputs of amplifiers A1 and A2 are applied to ground via resistors R1 and R2, respectively.

The outputs of amplifiers A1 and A2 are delivered to differential amplifier stage 38 which comprises a pair of high pass filters C3/R3, and C4/R4, and a differential amplifier A3. Specifically, the output of amplifier A1 is delivered to the inverting input of amplifier A3 via filter C3/R3, while the output of amplifier A2 is delivered to the noninverting input of amplifier A3 via filter C4/R4. The output of amplifier A3 is fed back to the inverting input thereof via resistor R5, while the noninverting input of amplifier A3 is fed to ground via resistor R6.

The output of differential amplifier stage 38 is applied to Butterworth filter 40 which comprises operational amplifier A4, resistors R7-R10, and capacitors C5 and C6. The output of amplifier A3 is applied to the noninverting input of amplifier A4 via resistors R7 and R8. The output of amplifier A4 is fed back to the node common to resistors R7 and R8 via capacitor C5, the output of amplifier of A4 also being fed back to the inverting input thereof via resistor R9. The noninverting and inverting inputs to amplifier A4 are applied to ground via capacitor C6 and resistor R10, respectively.

The output of the Butterworth filter taken at the output of amplifier A4 is delivered to amplifier stage 42 which comprises operational amplifier A5, resistors R11-R13 and capacitor C7. The output of the Butterworth filter is applied to the inverting input of amplifier A5 via capacitor C7 and resistor R11, the output of amplifier A5 being fed back to its inverting input via resistor R12. The noninverting input to amplifier A5 is applied to ground via resistor R13.

The output of amplifier A5 is delivered to the second Butterworth filter 44 which is substantially identical to Butterworth filter 40, resistors R14-R17 corresponding to resistors R7-R10, respectively, capacitors C8 and C9 corresponding to capacitors C5 and C6, respectively. Amplifiers A6 and A4 are substantially identical. The output of Butterworth filter 44 is applied to detector stage 46 and converter stage 48.

Detector stage 46 comprises operational amplifier A7, resistors R18-R20 and diodes D1 and D2. The output of Butterworth filter 44 is applied to the inverting input of amplifier A7 via resistor R18. The output of amplifier A7 is connected to cathode of diode D2, the anode of diode D2 being applied to the inverting input of amplifier A7. The output of amplifier A7 is also applied to the anode of diode D1, the cathode of diode D1 being applied to the inverting input of amplifier A7 via resistor R19. The noninverting input of amplifier A7 is applied to ground via resistor R20. The output of detector 46 is applied on the one hand to amplifier stage 50 and on the other hand to earphone circuitry 30.

Amplifier stage 50 comprises operational amplifier A8, resistors R21-R24 and capacitor C10. The output of detector 46 taken from the cathode of diode D1 is applied to the noninverting input of amplifier A8 via resistor R21. The output of amplifier A8 is fed back to the inverting input thereof via the wiper of variable resistor 22 and resistor R23. The inverting and noninverting inputs of amplifier A8 are applied to ground via resistor R24 and capacitor C10, respectively. The output from operational amplifier A8 is applied to the power LED display 10b, FIG. 2.

The output of amplifier A6 in Butterworth filter 44 is applied to the frequency-to-voltage conversion device 48 which comprises integrated circuit IC1, resistors R25, R26 and capacitors C11-C13. IC1 may be a National Semiconductor LM2907 Frequency-to-Voltage Converter. Pin 1 of IC1 receives the output of amplifier A6 via capacitor C11. Pin 2 of IC1 is applied to groun via capacitor C12. Pin 3 of IC1 is applied to ground via resistor R25 and capacitor C13 connected in parallel. Pins 4 and 7 are tied together and provide the input signal to the frequency LED display 12b, FIG. 2. Pins 4 and 7 are also applied to ground via resistor R26. The positive supply voltage is applied to pins 5 and 6, and pin 8 is applied directly to ground.

The output of detector 46 taken at the cathode of diode D1 is applied to earphone circuitry 30 which comprises operational amplifier A9, integrated circuit IC2, resistors R27-R30, capacitors C14, C15, switch 18, and earphone jack 16. The output of detector 46 is applied to the noninverting input of amplifier A9 via resistor R27 and switch 18. The output of amplifier A9 is fed back to the inverting input thereof via resistor R28. The inverting and noninverting inputs of amplifier A9 are applied to ground via resistor R29 and capacitor C14, respectively. The output of amplifier A9 is applied to a voltage controlled oscillator which is included in IC2, a CD4046 Phase-Locked Loop. The signal from amplifier A9 is connected to pin 9 of integrated circuit IC2. Pins 6 and 7 of IC2 are connected to each other via capacitor C15, pin 11 of IC2 being applied to ground via resistor 30. Finally, pin 4 of IC2 is applied to ground through earphone jack 16.

The Power Spectrum Analyzer circuitry illustrated in FIG. 4 operates as follows. The bipolar signal at input 24 is applied to input stage amplifiers A1 and A2 which provide a very high impedance, on the order of 10¹² ohms, in order to isolate the circuitry from the patient. The high pass filters comprising resistor/capacitor pairs R1/C1, R2/C2, R3/C3, R4/C4, and R11/C7, are designed to have a low frequency cutoff at approximately 2-4 Hz. The outputs of amplifiers A1 and A2 are applied to amplifier A3 which functions as a differential preamp having a gain of approximately 1000. The amplified output from amplifier A3 is applied to second order Butterworth filter 40, the components of which are designed to provide a rapid roll-off in frequency response above 12 Hz. The output from Butterworth filter 40 is applied to amplifier A5 which functions to amplify the signal input thereto by approximately 82. The output of amplifier A5 is applied to second order Butterworth filter 44 having characteristics substantially identical to that of Butterworth filter 40. Thus, the signal available at the output of amplifier A6 represents the filtered and amplified differential signal produced by the bipolar electrode 24. The filters combine to produce lower and upper filter skirts at around 4 and 12 Hz, respectively. However, signals having frequencies above and below the pass band, although attenuated, will still exist to some degree. As will be discussed in more detail below, the frequency detection is based on zero crossings of the signals and is thus independent of amplitude. Therefore, a band of signal frequencies broader than the filter pass band may be examined for spectral content.

The output from amplifier A6 is level detected in detector 46 which serves to half-wave rectify the signal input thereto. The output of detector 46 is applied to amplifier stage 50 which provides an adjustable gain for the power LED display 10b, variable resistor R22 corresponding to control knob 4, FIG. 1. By adjusting resistor R22 the LED response may be adjusted as desired. The LED display provides a simple indication of the power of the brain waves.

The output of amplifier A6 is also applied to frequency-to-voltage converter 48 which comprises IC1 and associated circuitry. IC1 provides three functions. Recalling that frequencies over a broad band will be applied to the converter 48 (the frequencies outside the pass band being attenuated), it is necessary to eliminate minute system noise signals so that they do not affect the brain wave indications. This is accomplished in IC1 with a Schmitt trigger device having a preset hysteresis which provides a threshold voltage swing through which the signal must pass in order for it to be analyzed further. In this manner, the system may be easily adjusted to respond to signals of any frequency at or above a given amplitude. The output of the Schmitt trigger is applied to a zero crossing detector which functions to provide an output signal whenever the Schmitt trigger changes state. Finally, this zero crossing signal is applied to a frequency-to-voltage converter, the frequency-to-voltage converter producing a voltage proportional to the frequency of the zero crossing signals. Converter 48 can thus provide the frequency information over a broad range of frequencies (on the order of 0-20 Hz) to frequency LED display 12b, FIG. 2.

The output of detector 46 is applied to earphone circuitry 30 comprising amplifier A9, integrated circuit IC2, and associated circuitry. The detected power signal is suitably amplified in amplifier A9 and delivered to the voltage controlled oscillator input at pin 9 of IC2. The voltage applied to pin 9 of IC2 is proportional to the amplitude of the detected signal. The voltage controlled oscillator in IC2 functions to convert this voltage to an audio frequency proportional to the voltage. The frequency signal which is taken from pin 4 of IC2 is applied to earphone jack 16. Thus, by listening to the frequency of the signal available at the earphone jack, and by selecting the left or right hemisphere of the brain, the operator can listen to any tonal differences between the left and right hemispheres, to thereby alert the operator to any differences in brain wave signal power between the left and right hemispheres of the brain.

Thus, the dual channel monitoring device provides the following major advances over the prior art techniques of monitoring brain waves. The first is a rapid and simple indication of brain dysfunction. The signal power level as indicated in the output from side-by-side linear LED displays 10a and 10b, and the signal frequencies as indicated in the output of side-by-side linear LED displays 12a and 12b may readily be monitored by absolute response, since the output is in the form of a "bar graph" which does not require the operator to read and interpret an alphanumeric display or chart record. Further, the operator can quickly and easily determine the existence of any type of unilateral hemisphere dysfunction since such dysfunction will produce a visible difference between the outputs of power displays 10a and 10b and/or frequency displays 12a and 12b. The detection of unilateral hemisphere dysfunction is further enhanced by providing an audible indication of signal power from earphone jack. By switching back and forth using switch 18, the operator can readily detect differences in hemisphere response.

Still further, the monitoring device of the co-pending application is advanced over the prior art devices since it provides an indication of, and tracks the frequency of the signal of interest over a broad range of frequencies. The operator can therefore monitor both amplitude and frequency without having to know or select a priori the frequency of interest to the exclusion of other frequencies. This is accomplished using a minimal amount of additional circuitry and provides the capability of monitoring the patient's condition throughout a wide range of operative procedures without further adjustment by the operator.

FIG. 5 is a schematic illustration of system test circuit 36 and impedance test circuit 34, in accordance with the present invention, connected to the Power Spectrum Analyzer 28, FIG. 2. System test circuit 36 comprises integrated circuit IC3 and associated circuitry comprising resistors R31-R44 and capacitors C16-C18. IC3 is a timing device such as the LM555 timer and provides a pulsating DC signal having predetermined amplitude and frequency characteristics on signal output line 52. Pins 4 and 8 of IC3 are directly applied to the supply voltage, while pin 7 receives the supply voltage via resistors R31 and R32. Pin 6 and pin 2 receive the supply voltage via resistor R31. Pin 5 is connected to pin 6 via capacitors C16 and C17. Pin 1 is directly applied to ground, and is connected to pin 5 via capacitor C16. Pin 6 is applied to switch 14a via capacitor C18. The signal output from IC3 taken from pin 3 is applied to signal output line 52 via resistor R33, and is applied to ground via resistors R33 and R34. The output signal on line 52 is applied to left and right Power Spectrum Analyzer channels 26 and 28 via resistors R36/R37 and R38/R39, respectively, at the bipolar inputs 22 and 24, respectively, resistors R38 and R39 being connected to the noninverting inputs to amplifiers A1 and A2, respectively. The noninverting inputs to amplifiers A1 and A2 are applied to ground via resistors R35A and R35b, respectively, in series with double pole switch 14b. In FIG. 5, the input stage 35 for Power Spectrum Analyzer 28 is shown in detail, the application of signal to Power Spectrum Analyzer 26 being identical to that for Power Spectrum Analyzer 28.

The output signal on line 52 is applied to the inverting input of amplifier A10 via switch 14c and resistor R40. The output of amplifier A10 is fed back to the inverting input thereof via resistor R41, the noninverting input of amplifier A10 being applied to ground via resistor R42. The output of amplifier A10 is further applied to the input terminal connected to input stage amplifier A2 via resistor R43, the same input terminal to amplifier A2 receiving the signal on line 52 via resistor R39. In an identical manner, the output of amplifier A10 is also applied to the input stage of Analyzer 26 via resistor R44.

Impedance test circuitry 34 for Power Spectrum Analyzer 28 is also illustrated in detail in FIG. 5. The details of impedance test circuitry 32 for Power Spectrum Analyzer 26 one identical to that shown in FIG. 5 and have therefore been omitted. Respective outputs from amplifiers A1 and A2 are applied to amplifier 38 (FIGS. 3 and 4), and to a pair of identical comparator circuits 54 and 56. Comparator circuit 54 includes an amplifier A11 which receives at is noninverting input the output from input stage amplifier A1 via capacitor C19. The output of amplifier A11 is fed back to the inverting input thereof via resistor R45. The inverting and noninverting inputs to amplifier A11 are applied to ground via resistors R46 and R47, respectively. The output of amplifier A11 is applied to the inverting input of amplifier A12 via capacitor C20, the output of amplifier A12 being applied to the anode of LED D3, the cathode of which is connected to a negative supply voltage. The inverting input to amplifier A12 is applied to ground via resistor R48, the noninverting input being applied to the positive supply voltage via resistor R49 on the one hand and being applied to ground via variable resistor R50 on the other hand. The comparator circuit 56 is identical to circuit 54, capacitors C21, C22 corresponding to capacitor C19 and C20, amplifiers A13, A14 corresponding to amplifiers A11, A12, resistors R51-R56 corresponding to resistors R45-R50 and diode D4 corresponding to diode D3, respectively.

Operation of the system test circuitry 36 and impedance test circuitry 34 will now be discussed. IC3 and associated circuitry produce a pulsed (square wave) DC output at about 23-24 millivolts on signal line 52, the frequency of the output being either 120-150 Hz when switches 14a and 14c are open, or about 9 Hz when switches 14a and 14c are closed. Switches 14a-14c are controlled in unison, switches 14a and 14c embodied in a quad analog switch such as the CD4016, switch 14b being embodied in a low resistance analog switch such as the CD4066. Assuming that switches 14a-14c are open, the 120-150 Hz signal is applied to the input terminals 24 of input stage 35 via resistors R38 and R39. As shown in FIG. 5, current paths i₁ and i₂ will be established from each of the bipolar electrodes to the ground electrode. Thus, as the impedance through paths i₁ and i.sub. 2 is decreased, the amplitude of the signal applied to the input stage 35 will be decreased, while as the impedance of the paths i₁ and i₂ increases, the amplitude of the signal applied to the input stage 35 accordingly increases. The signal output from stage 35 is applied to the comparator circuits 54 and 56, where they are subject to amplification in amplifiers A11 and A13, respectively, and are compared with a reference level at amplifiers A12 and A14, respectively. If the signals applied to the inverting inputs of amplifiers A12 or A14 are of sufficient magnitude, indicating that the impedance along paths i₁ or i₂ is above a predetermined level (typically 5K ohms), the outputs of amplifiers A12 or A14 will be drawn negative to thereby turn the associated diode D3 or D4 off to indicate that one of the respective paths i₁ or i₂ presents an impedance which is prohibitively high. The impedance test signal may be applied during monitoring since the 120-150 Hz signal is far outside the pass band provided by the Butterworth filters, and since any brain wave signals (around 50 uV) present at the inputs to stage 35 will have a negligible effect on the 23-24 mV impedance signal.

The system test circuitry 36 further provides for the injection of a signal having known frequency and amplitude characteristics into the left and right Power Spectrum Analyzers 26 and 28 so that the operator may readily determine whether the associated circuitry is operating properly by simply pushing a button. When pushbutton 14 is activated by the operator, switches 14a-14c, FIG. 5 are closed. Closure of switch 14a adds capacitor C18 into the timer circuit of IC3 to thereby provide a signal on line 52 of approximately 23-24 mV at 9 Hz, the approximate frequency of a typical brain wave signal. This output signal is applied to input stage 35 via resistors R38 and R39, and is also applied to inverting amplifier A10 via switch 14c. Amplifier A10 and resistors R40-R42 function to invert the signal on line 52 and multiply the amplitude thereof by 2. Thus, by combining the output of amplifier A10 with the signal applied to the input of amplifier A2, amplifier A2 effectively receives a signal equal in amplitude but opposite in polarity to that on line 52. The effect of the signal produced by amplifier A10 on the input to amplifier A1 is substantially reduced by the additional resistor R39. Therefore, the signals applied to the inputs of amplifiers A1 and A2 are substantially of equal amplitude and opposite polarity. Since the signals on line 52 and from amplifier A10 are much greater in magnitude than any brain wave signals present at the input to stage 35, their effect on the system test is negligible. Finally, by closing switch 14b, low impedance (around 30 ohms) resistors R35a and R35b are added to the circuit to thereby reduce the amplitude of the signals applied to stage 35 to approximately 50 uV, a typical brain wave amplitude. By applying such signals of known amplitude, polarity and frequency to each of the Power Spectrum Analyzers 26 and 28, the power and frequency displays 10 and 12 should produce a predetermined indication. In the event that this indication is not so produced, the operator is immediately aware that a portion of the system is malfunctioning. Of course, control knob 4 which controls variable resistors R22 must be set to a predetermined position for the test.

Each of the linear LED arrays 10a, 10b, 12a and 12b may be comprised of the circuitry illustrated in FIG. 6. With reference to FIG. 6, an associated one of the signals to be displayed from Power Spectrum Analyzers 26 and 28 is applied to pin 5 of IC4 which is an LED driver such as the LM3914. Pins 6 and 7 are connected directly to each other, and are connected to pin 8 via resistor R58. Pin 8 is further applied to ground via variable resistor R57. Pins 4 and 2 are connected directly to ground, while pin 3 receives the supply voltage. Pins 1, and 18-10 from IC4 are applied to the cathodes of a linear array of diodes in display device 58, the positive supply voltage being applied to the anodes of the LED's.

IC4 operates to provide a path to ground for the cathodes of a number of the LEDs in display 58 to thereby illuminate the associated diodes, the number of diodes provided with the cathode path to ground being proportional to the voltage supplied to pin 5 of IC4. Thus, a direct read-out of the amplitude of the output signals is provided as a "bar graph" type display.

The Table below lists approximate component values which have been successfully used in the brain wave monitoring device. These values are only used as examples for providing a working device and various alternatives to these values will be apparent to those skilled in the art.

While the preferred embodiments have been described with reference to the specification, the scope of the invention will be defined in accordance with the following claims.

                  TABLE                                                            ______________________________________                                                  Resistors                                                             Element No.                                                                             (ohms)   Capacitors (μf)                                                                         Amplifiers                                                                             IC's                                     ______________________________________                                         1        10M      .1          LF 353   LM2907                                  2        10M      .1          LF 353  CD4046                                   3        1K       47          LF 353  LM555                                    4        1K       47          MC 1458 LM3914                                   5        1M       10          LF 353                                           6        1M       10          MC 1458                                          7        1K       10          MC 1458 -8                                                                             1K 10 MC 1458                            9        27K      10          741                                              10       47K      22          741                                              11       1K       10          741                                              12       100K     .02         741                                              13       1K       1.5         741                                              14       1K       4.7         741                                              15       1K       .002                                                         16       27K      .02                                                          17       47K      .1                                                           18       10K      1.5                                                          19       10K      .1                                                           20       15K      .1                                                           21       100K     .1                                                           22       10K      .1                                                           23       4.7K     10                                                           24       1K                                                                    25       1M                                                                    26       10K                                                                   27       22K                                                                   28       33K                                                                   29       1K                                                                    30       220K                                                                  31       47K                                                                   32       27K                                                                   33       1M                                                                    34       4.7K                                                                  35a,b    30                                                                    36       100K                                                                  37       100K                                                                  38       100K                                                                  39       100K                                                                  40       1K                                                                    41       2.2K                                                                  42       1K                                                                    43       10K                                                                   44       10K                                                                   45       1M                                                                    46       1K                                                                    47       10K                                                                   48       10K                                                                   49       100K                                                                  50       500                                                                   51       1M                                                                    52       1K                                                                    53       10K                                                                   54       10K                                                                   55       100K                                                                  56       500                                                                   57       1K                                                                    58       270                                                                   ______________________________________                                     

What is claimed is:
 1. Apparatus comprising a device for monitoring brain wave signals detected by at least one electrode adapted to be placed on a patient's head, said brain wave signals having known frequency and amplitude ranges, and circuitry for use with said device for detecting impedance properties of at least one current path on said patient's head, said circuitry comprising:means for applying a time varying signal having an amplitude and frequency substantially greater than said known amplitude and frequency ranges to said at least one electrode, a first portion of said time varying signal flowing into said at least one current path on said patient's head, and a second portion of said time varying signal being injected into said device; amplifier means for amplifying said second portion of said time varying signal to produce a first voltage proportional to the amplitude of said second portion of said time varying signal; means for providing a second voltage of predetermined amplitude; differential amplifier means receiving at a first input said first voltage, and at a second input said second voltage, for producing an output voltage proportional to the difference of said first and second voltages, and indicator means responsive to the voltage level of said output voltage for providing an indication of the impedance properties of said at least one current path, whereby the impedance properties of said at least one current path can be monitored concurrently with the use of said device for monitoring brain wave signals.
 2. The apparatus of claim 1 wherein said time varying signal is a pulsating DC signal.
 3. The apparatus of claim 2 wherein said pulsating DC signal is a square wave signal having a duty cycle of approximately 50%, a frequency of at least 120 Hz, and an amplitude of at least 20 millivolts.
 4. The apparatus of claim 3 wherein said indicator means comprises an LED.
 5. Apparatus comprising a device for monitoring brain wave signals detected at the output of at least one electrode adapted to be placed on a patient's head, said brain wave signals having known frequency and amplitude ranges, and circuitry for testing the operation of said device without removing said at least one electrode from said patient's head, said circuitry comprising:means for producing a time varying signal having a predetermined frequency within said known frequency range, and an amplitude substantially above said known amplitude range; means for connecting said output of said electrode and said time varying signal in common to said device for providing a common input signal having an amplitude and frequency substantially determined by the amplitude and frequency of said time varying signal; and means for reducing said amplitude of said common input signal to a predetermined amplitude in said known amplitude range, whereby said device can be tested in accordance with said predetermined amplitude and frequency without removing said at least one electrode from said patient's head, said brain wave signals having a negligible effect on said common input signal.
 6. The apparatus of claim 5 wherein said time varying signal is a square wave signal having a duty cycle of approximately 50%, a frequency of about 9 Hz, and an amplitude of at least 20 millivolts.
 7. The apparatus of claim 5 wherein said means for reducing said amplitude comprise means for resistively coupling said common input signal to ground.
 8. Apparatus comprising a device for monitoring brain wave signals detected at the output of at least one electrode adapted to be placed on a patient's head, said brain wave signals having known frequency and amplitude ranges, and circuitry for selectively (i) detecting impedance properties of at least one current path on said patient's head, and (ii) testing the operation of said device without removing said at least one electrode from said patient's head, said circuitry comprising:signal generator means for selectively producing at an output thereof (i) an impedance signal or (ii) a system test signal, said impedance signal having an amplitude and frequency substantially greater than said known amplitude and frequency ranges, said system test signal having a predetermined frequency within said known frequency range and an amplitude substantially above said known amplitude range; switching means for selectively switching said signal generator means to produce either said impedance signal or said system test signal; means for applying said output of said signal generator means and said output of said electrode in common to said device for providing a common input signal having an amplitude and frequency substantially determined by the amplitude and frequency of said output of said signal generator, a portion of said common input signal being injected into said device; amplifier means for amplifying said portion of said common input signal to produce a first voltage proportional to the amplitude of said portion of said common input signal; means for providing a second voltage of predetermined amplitide; differential amplifier means receiving at a first input said first voltage, and at a second input said second voltage, for producing an output voltage proportional to the difference of said first and second voltages, indicator means responsive to the voltage level of said output voltage is providing an indication of the impedance properties of said at least one current path, whereby the impedance properties of said at least one current path can be monitored concurrently with the use of said device for monitoring brain wave signals, when said signal general means is producing said impedance signal; and means functionally coupled to said switching means for reducing said amplitude of said common input signal to a predetermined amplitude in said known amplitude range when said signal generator means is producing said system test signal, whereby said device can be tested in accordance with said predetermined amplitude and frequency without removing said at least one electrode from said patient's head, said brain wave signals having a negligible effect on said common input signal. 